Method of making molybdenum composite materials

ABSTRACT

A COMPOSITE MATERIAL IN WHICH THE PRIMARY BY VOLUME CONSTITUENT IS MOLYBDENIUM HAVING A LOW GAS CONTENT FOR USE AS A CURRENT CARRYING MATERIAL. THE COMPOSITE MATERIAL CONSISTS ESSENTIALLY OF A SUBSTANTIALLY CONTINUOUS SKELETON OF INTEGRALLY BONDED MOLYBDENUM PARTICLES, THE VOIDS OF WHICH ARE FILLED BY A SUBSTANTIALLY MUTUALLY MATRIX OF A METAL ALLOY WHICH IS SUBSTANTIALLY MUTUALLY INSOLUBLE AND SUBSTANTIALLY NON-REACTIVE WITH MOLYBDENUM. THE ALLOY MATRIX INCLUDES AT LEAST ONE HIGHLY CONDUCTIVE METAL AND ANOTHER METAL WHICH IS LESS CONDUCTIVE WHEREIN THE LESS CONDUCTIVE METAL TENDS TO CONCENTRATE AT THE INTERFACE BETWEEN THE MOLYBDENUM AND THE ALLOY MATRIX.

March 30, 1971' R. H. KROCK 3,573,037

METHOD OF MAKING MQLYBDENUM COMPOSITE MATERIALS Original Filed Jan.' 22, 1958 CONTACTING MOLYBDENUM POWDER BODY WITH A METAL AND A WETTING AGENT INFILTRATING MOLYBDENUM POWDER BODY WITH A METAL AND A WETTING AGENT IN A VACUUM ATMOSPHEIRE INVENTORS EDWARD J. ZDANUK RICHARD H. KROCK BY H 3 ATTORNEY United States Patent US. Cl. 75-208 8 Claims ABSCT OF THE DISCLOSURE A composite material in which the primary by volume constituent is molybdenum having a low gas content for use as a current carrying material. The composite material consists essentially of a substantially continuous skeleton of integrally bonded molybdenum particles, the voids of which are filled by a substantially continuous matrix of a metal alloy which is substantially mutually insoluble and substantially non-reactive with molybdenum. The alloy matrix includes at least one highly conductive metal and another metal which is less conductive wherein the less conductive metal tends to concentrate at the interface between the molybdenum and the alloy matrix.

This is a division of application Ser. No. 699,658, filed Jan. 22, 1968, now Pat. No. 3,407,048.

The present invention relates to powder metallurgy and, more particularly, to improved means and methods of providing composite materials having a low gas content and low content of materials which may release gas for use as contact materials in current interrupter devices such as switching devices and the like which may operate in a vacuum atmosphere.

Molybdenum is used in electrical contact materials because of its inherent characteristics of hardness and resistance to arcing. Due to the inherent characteristics of molybdenum, molybdenum contacts are less likely to pit than contacts fabricated from copper, silver or gold. However, pure molybdenum contacts possess high electrical resistance which lowers the efiiciency and reliability of the molybdenum contact materials. Pure copper, silver, or gold contact materials have good thermal and conductivity characteristics but are likely to experience arc erosion and contact welding.

It is thought that a contact material composed essentially of a continuous skeleton of bonded molybdenum particles integrally joined with a continuous matrix material which has good thermal and electrical properties and which is substantially mutually insoluble and substantially non-reactive with molybdenum might make advantageous use of the several outstanding characteristics of the constituents. For example, if the contact material is composed of a matrix material selected from the group consisting of copper, silver and gold which is alloyed with a reactive metal selected from the group consisting of titanium and zirconium, the alloy being substantially mutually insoluble and substantially non-reactive with molybdenum, it is thought that the composite material will have good current carrying and thermal conductivity properties, hardness, resistance to arc erosion and superior antiweld properties.

Composites of molybdenum and copper may be prepared by pressing the mixed metal powders into a required shape and subsequently sintering the compacted powders in a hydrogen atmosphere above the melting point temice perature of copper, preferably between 1250 and 1350 C. The hydrogen atmosphere appears to act as a flux and the molten copper wets the molybdenum particles and cements the particles together. The copper infiltrant imparts strength and ductility to the molybdenum powder body and also provides the resultant body with higher current carrying capability and thermal conductivity. However, it is noted that the above process makes no attempt to reduce the gas content or content of materials that may release gas during the operation of the material in switching devices operating in a vacuum. Therefore, it would appear that the molybdenum-copper composite fabricated by the above process would be unacceptable for use as a contact material in a vacuum environment due to its high gas content and content of materials which may release gas during the operation of the composite material in a vacuum environment. It is thought that the gas content and the content of materials which may release gas in such a contact material is in excess of 50 parts per million.

It is thought that in order to reduce the gas content of the contact material, infiltration should be carried out in a vacuum atmosphere. A copper melt shows substantially no penetration into a molybdenum powder body in a vacuum using standard metallurgical techniques. It is thought that the lack of penetration of copper into the molybdenum powder body in a vacuum atmosphere may be due to the unfavorable surface energies present in the vacuum atmosphere.

If there is insolubility between the metals as is the case between molybdenum and copper, and if the wetting is poor, an auxiliary agent influencing the surface energies in the desired direction is required in order to achieve infiltration of a powder body of one metal with a melt of a second metal or metal alloy.

In the present invention, by utilizing small amounts of an auxiliary agent or reactive metal selected from the group consisting of titanium and zirconium and by using vacuum infiltration techniques, a melt of metal selected 7 from the group consisting of copper, silver and gold alloyed with a small but significant percent of an auxiliary agent or reactive metal selected from the group consisting of titanium and zirconium infiltrates the voids of the molybdenum powder body so as to provide a substantially voidless composite. It is thought that the agent either raises the surface energies of the melt or of the solid, or lowers the surface energies of the interface between the melt and the solid thereby favoring infiltration of the melt into the molybdenum powder body. It is thought that the vacuum serves the dual functions of assisting penetration of the alloy melt into the molybdenum powder body and of decreasing the content of all gases present. As indicated before, in order for the composite material to be acceptable for operation in a vacuum atmosphere the composite material should contain a low gas content.

Therefore, it is an object of the present invention to provide a composite material suitable for use as a current carrying material in a vacuum atmosphere.

Another object of the present invention is to provide a means and method of fabricating a composite contact material using vacuum infiltration techniques.

A further object of the present invention is to provide means and methods of fabricating a composite contact material having high electrical and thermal conductivity properties combined with low erosion under arcing and high resistance to welding.

A further object of the present invention is to provide an alloy which includes an auxiliary agent for wetting a molybdenum skeleton, that is ductile, that has high electrical and thermal conductivity, and has a melting point temperature which is lower than that of molybdenum.

A further object of the present invention is to provide a composite material of a substantially continuous skeleton of integrally bonded molybdenum, the voids of which are filled by a substantially continuous matrix of an alloy which is substantially mutually insoluble and substantially non-reactive with the molybdenum for use as an electrical contact material wherein the concentration of a less conductive metal in the alloy matrix has a higher concentration in the molybdenum interface region than in the region between the molybdenum and the alloy matrix thereby raising the overall conductivity of the alloy matrix.

Another object of the present invention is to provide a means and method of using an alloy which is substantially mutually insoluble and substantially non-reactive with molybdenum as infiltration stock for molybdenum powder bodies so as to allow substantially complete vacuum infiltration of the voids areas of molybdenum powder body thereby providing a composite contact material having a substantially continuous molybdenum skeleton integrally joined with a substantially continuous alloy matrix material.

Another object of the present invention is to provide a current carrying material consisting essentially of a metal alloy which is substantially mutually insoluble and substantially non-reactive with molybdenum, the molybdenum content of the material constituting the majority by volume percent of the material.

A further object of the present invention is to provide a current carrying material consisting of about 55 to 88% by volume molybdenum and about 45 to 12% by volume of an infiltrant which is substantially mutually insoluble and substantially non-reactive with the molybdenum.

The present invention, in another of its aspects, relates to the novel features of the instrumentalities of the invention described herein for teaching the principal object of the invention and to the novel principles employed in the instrumentalities, whether or not these features and principles may be used in the said object and/or in the said field.

With the aforementioned objects enumerated, other objects will be apparent to those persons possessing ordinary skill in the art. Other objects will appear in the following description and in the appended claims.

In the drawings:

FIG. 1 illustrates the several steps involved in the method of making the current carrying material.

FIG. 2 illustrates the current carrying material.

Generally speaking, the present invention relates to a composite of a substantially continuous skeleton of integrally bonded molybdenum particles, the voids of which are filled by a substantially continuous matrix of an alloy which is substantially mutually insoluble and substantially non-reactive with the molybdenum. The molybdenum content of the composite constitutes the majority by volume percent of the composite.

The method of the present invention relates to contacting a porous sintered or unsintered molybdenum body with an alloy which contains an agent for wetting the molybdenum particles so that the alloy substantially completely infiltrates the body in a vacuum atmosphere.

More particularly, the means and methods of the pres ent invention relate to fabricating a current carrying material consisting essentially of a continuous skeleton of integrally bonded molybdenum particles, the voids of which are filled by a substantially continuous matrix of a metal alloy which is substantially mutually insoluble and substantially non-reactive with molybdenum. The metal alloy consists essentially of a first metal selected from the group consisting of copper, gold and silver and a small but significant addition of a second metal selected from the group consisting of titanium and zirconium. Of the first group of metals copper is preferred. Of the second group of metals titanium is preferred.

The molybdenum powder has a particle size ranging from preferably about 1 to 50 microns. The molybdenum particle size may vary in accordance with the desired porosity of the molybdenum powder body. It should be understood that molybdenum powder having a larger or smaller particle size may be used. The powder is compacted in a suitable mold under a pressure sufficient to provide green compacts with sufiicient strength to be handled and with densities which will yield desired composite composition. Compacting pressures ranging from 10 to 30 tons per inch squared resulted in green compacts with desired strength and density. The compact densities range from about 5585% of theoretical density. The molybdenum body may be sintered in any suitable non-oxidizing atmosphere such a hydrogen, argon, dissociated ammonia, or vacuum atmosphere for about 1 to 15 hours or more at a temperature of about 13001800 C. The lower the sintering temperature, the longer the sintering time. However, sintering of the compact is not a necessary prerequisite to successful infiltration thereof by the alloy melt. The compact, sintered or unsintered, is placed in a vacuum atmosphere. The vacuum atmosphere may have a pressure of about 1X10' torr or harder vacuum. A vacuum pressure of about 5 X l0 torr or harder is preferred. The molybdenum powder body is contacted with the alloy which is substantially mutually insoluble and substantially non-reactive with the molybdenum body. The molybdenum body, sintered or unsintered, is infiltrated in the vacuum environment with the alloy metal at a temperature sutficiently high to yield a free flowing melt.

The infiltration temperature contemplated is about 1 1200 C. Attempting infiltration of the body with an alloy composed of the metals contemplated at a temperature below about 1100 C. poor fluidity of the melt is experienced. Attempting infiltration of the body at a temperature in excess of 1200 C. the melt appears to suffer excessive evaporation.

The molybdenum content of the composite material is about 55 to 88% by volume of the total volume of the composite. It is thought that a molybdenum content of less than 55% by volume leads to undesirable contact welding and are erosion during operation of the contact material. It is thought that a molybdenum content of more than 88% by volume of the total volume of the constituents of the composite material results in an undesirable electrical conductivity characteristic of the material. It is thought that the titanium or zirconium alloyed with either copper, gold or silver should not exceed about 0.4% by volume or be less than 0.01% by volume of the total volume of the constituents of the composite. For example, if the reactive metal content of the alloy exceeds about 0.4% by volume, it may result in an alloy matrix having an undesirable conductivity. Using less than about 0.01% by volume may result in seriously impeding the infiltration of the melt into the molybdenum body. The copper, gold or silver content of the total volume of the composite material is about 44.9 to 11.9% by volume.

It is believed that an electron probe analysis of a molybdenum-alloy matrix region will show segregation or high concentration of the titanium or zirconium constituent of the alloy in the interface region between the molybdenum and the alloy metal. Since titanium and zirconium are poorer conductors than either copper, gold or silver, the overall conductivity of the alloy matrix should be raised.

FIG. 1 shows the several steps in the method used to fabricate the composite illustrated in FIG. 2.

FIG. 2 illustrates a molybdenum-copper-titanium composite 10 consisting of the sintered porous skeleton of molybdenum. The voids of the skeleton are filled by a continuous network of an alloy of copper-titanium 12. The copper-titanium alloy is integrally bonded with the molybdenum skeleton.

The following Examples 1 to 3 are illustrative of the preparation of molybdenum-copper-titanium or of a molybdenum-silver-titanium composite contact material fabricated using the vacuum infiltration technique described above.

EXAMPLE 1 A molybdenum body vacuum infiltrated with an alloy of copper-titanium, the composite consisting essentially of about 72.3% by volume molybdenum, 27.5% by volume copper, the remainder titanium.

Powdered molybdenum having a particle size of about 45 microns or less is pressed by any suitable means such as by an automatic press at a pressure of about 12 tons per square inch to provide a green compact sturdy enough to be handled. The green compact has a compact density of about 73% of theoretical density. The green molybdenum compact is then sintered for about 15 hours at a temperature of about 1300 C. in a hydrogen atmosphere. The sintered compact is contacted with an alloy of copper-titanium in a vacuum atmosphere. The sintered compact and contacting alloy are heated at a temperature of about 1200 C. for about 30 minutes at a pressure of about 2 10- torr. The porous molybdenum body is infiltrated with the copper-titanium alloy. The resultant composite is a dense structure having high electrical and thermal conductivity combined with high resistance to deformation under pressure and has a low gas content, that is, a gas content and content of materials which may release gas of no more than 10 p.p.m.

EXAMPLE 2 A molybdenum powder body vacuum infiltrated with an alloy of copper-titanium. The composite consists essentially of about 76.5% by volume molybdenum, about 23.3% by volume copper, the remainder titanium.

Molybdenum particles having a particle size of about 45 microns or less is pressed by any suitable means such as by an automatic press at a pressure of about tons per square inch. The compact has a density of about 76% of theoretical density. The green compact is subjected to sintering for 7 hours at a temperature of about 1375 C. The sintered compact has a density of about 76.5% of theoretical density. The sintered compact is contacted with an alloy of copper-titanium in a vacuum atmosphere. The sintered compact and contacting alloy are heated for 30 minutes at a temperature of about 1200 C. at a pressure of about 2 10 torr. The resultant composite of molybdenum-copper-titanium is a dense structure having high electrical and thermal conductivity characteristics combined with high resistance to deformation under pressure and low gas content, that is, a gas content and content of materials which may release gas of no more than 10 p.p.m.

EXAMPLE 3 A molybdenum-silver-titanium composite material may be prepared using the procedure of Example 2. The proportions of the materials of the composite approximate the proportions recited above.

It is thought that the molybdenum powder body or the sintered molybdenum body may be coated with a metal selected from the group of titanium and zirconium by electrolytic or vapor phase plating, impregnation of the coated body could be carried out successfully with essentially pure (unalloyed) copper. Vacuum infiltration should occur as long as either zirconium or titanium is at the boundary. The amount of the plated zirconium or titanium will be such that its composition would amount to about 0.4 to 0.01% by volume of the total volume of the constituents.

The present invention is not intended to be limited to the disclosure herein, and changes and modifications may be made by those skilled in the art without departing from the spirit and the scope of the present invention. Such modifications and variations are considered to be Within the purview and scope of the present invention and the appended claims.

Having thus described our invention, we claim:

1. A method of making a composite material comprismg:

contacting a porous molybdenum body of compacted particles with an alloy formed from at least one electrically conducting metal selected from the group consisting of copper, silver and gold and at least one wetting metal selected from the group consisting of titanium and zirconium; and

heating said molybdenum body and said alloy in a vacuum environment so as to infiltrate said molyb denum body with said alloy, thereby forming a composite material.

2. A method of making a composite material as claimed in claim 1, wherein said metal is copper and said wetting agent is titanium.

3. A method of making a composite material as claimed in claim 1, wherein said molybdenum is about 55 to 88% by volume, said metal about 44.9 to 11.9% by volume and said wetting agent about 0.4 to 0.01% by volume.

4. A method of making a composite material as claimed in claim 1, wherein said molybdenum body is contacted with said wetting agent by electrolytic plating.

5. A method of making a composite material as claimed in claim 1, wherein said molybdenum body is contacted with said wetting agent by vapor phase plating.

6. A method of making a composite material as claimed in claim 1 which further comprises sintering said molybdenum body prior to contacting with said metal and said wetting agent.

7. A method of making a composite material as claimed in claim 6, wherein the sintering time is about 1 to 15 hours and the sintering temperature is about 1300" to 1800 C.

8. A method of making a composite material as claimed in claim 1, wherein the infiltration time is about 30 minutes, the infiltration temperature is about 1200 to 1400 C. and the infiltration pressure is about 1X10- to about 5X10 torr.

References Cited UNITED STATES PATENTS 3,303,026 2/1967 Zdanuk 208 3,303,559 2/1967 Holtzclaw 29-4205 3,337,338 8/1967 Krock 75208 LELAND A. SEBASTIAN, Primary Examiner B. H. HUNT, Assistant Examiner US. Cl. X.R. 75--200, 214, 224 

